In modern warfare, missiles are usually launched in the general direction of a target by a radar system that acquires the target, feeds initial coordinates about the target to the missile prior to launch, and then continues to transmit guidance signals to the missile enroute to the target. However, the missile system acquisition and guidance radar is subject to detection by passive tracking systems since its radar radiates energy. Passive direction finding systems have been used in the past as an "antiradiation" technique for direction finding and steering to a target. For small missiles, physical size necessitates the use of correspondingly small on-board antennas operating at high frequencies to reduce wavelength-associated dimensions. These concepts have prompted the development of small, highly accurate passive direction finding systems.
One type of passive direction finding system is a four-channel superheterodyne receiver. This type of receiver is typically used to provide broad radio frequency (RF) band coverage while using the selectivity of narrow band intermediate frequency (IF) components by tuning a local oscillator over the broad RF band. A common problem with this type of system is that it responds to RF spaced the intermediate frequency above and below the local oscillator frequency. Therefore, the response can provide ambiguous RF information if the upper and lower sidebands of the incoming signal are not identified.
Present methods of providing sideband identification make use of preselector filters, image rejection mixers and dithering of the local oscillator frequency. Preselector filters used for broad RF band applications usually require a high intermediate frequency. These filters are lossy and occupy extra space in the direction finding system. Further, the high intermediate frequency reduces the selectivity achievable. Thus, the overall system loses sensitivity because of the lossy preselector filters and the higher noise figures associated with the high intermediate frequency amplifiers.
Image rejection mixers are also lossy components and occupy more space than conventional mixers. Furthermore, image rejection mixers must also be used in each channel. However, if space is economized by using only a single channel, there will be a loss in both sensitivity and symmetry. Therefore, symmetrical arrangements, although requiring a greater amount of space, are used because of their tolerance of systematic errors since channel-to-channel tracking reduces the measurement error. Finally, techniques requiring the dithering of the local oscillator are unacceptable because of the time involved. This is especially true in many direction finding applications that require real-time information on every pulse.